In a focusing system of an imaging lens, there are a system of moving a total of a lens system and a system of moving a portion of a lens system. In the case of a telephoto lens, in comparison with a standard lens, a lens system becomes large-sized and also a weight thereof becomes heavy and therefore, the system of moving a portion of a lens system is frequently adopted.
For example, JP-A-2002-55275 discloses a telephoto lens having a half-field angle of about 4° through 8° (a focal length of about 150 mm through 300 mm in a 35 mm film) of an inner focus type constituted by three lens groups having a positive, a negative, a positive refracting power successively from an object side, in which the focusing is performed by moving the second lens group having the negative refracting power. The first lens group is constituted by 3 lenses of a positive first lens, a positive second lens and a negative third lens successively from the object side. The second lens group is constituted by 2 lenses of a negative fourth lens and a positive fifth lens successively from the object side. For achromatism, a glass material having a large ν value (Abbe number) is adopted for the positive first and second lenses, and a glass material having a small ν value is adopted for the negative third and fourth lenses.
The telephoto lens disclosed in JP-A-2002-55275 is constituted by an optical system of the inner focus type which is comparatively excellent in an optical function from an infinite object point over to a close range and is advantageous in small-sized formation. Here, as a general method of correcting chromatic aberration, similar to the telephoto lens disclosed in JP-A-2002-55275, glass having a large ν value is used for a positive lens and glass having a small ν value is adopted for a negative lens. An amount of secondary chromatic aberration which remains at this occasion is determined by a magnitude of anomalous dispersion of the glass adopted for the positive lens. Although in order to reduce the secondary chromatic aberration, much of the anomalous dispersion glass may be adopted for the positive lens in the lenses, according to the method, there poses a problem that a number of elements of the anomalous dispersion glass is increased, which is disadvantageous in view of cost.